G=1x4 + 4x7 + ... +94x97
A=1x4+4x7+7x10+.........+94x97
tìm A
Tính
3/1x4 + 3/4x7 + 3/7x10 + ... + 3/94x97
\(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+...+\frac{3}{94.97}\)
\(=1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{94}-\frac{1}{97}\)
\(=1-\frac{1}{97}\)
\(=\frac{96}{97}\)
Bài làm:
Ta có: \(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+...+\frac{3}{94.97}\)
\(=1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{94}-\frac{1}{97}\)
\(=1-\frac{1}{97}\)
\(=\frac{96}{97}\)
Ta có \(\frac{3}{1.4}+\frac{3}{4.7}+...+\frac{3}{94.97}=1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+...+\frac{1}{94}-\frac{1}{7}=1-\frac{1}{7}=\frac{6}{7}\)
P/S : Dấu "." là dấu "x" nha !
s=1/1x4+1/4x7+1/7x10+...+1/94x97+1/97x100
\(S=\frac{1}{1\times4}+\frac{1}{4\times7}+\frac{1}{7\times10}+...+\frac{1}{94\times97}+\frac{1}{97\times100}\)
\(S=\frac{1}{3}\times\left(\frac{1}{1}-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{94}-\frac{1}{97}+\frac{1}{97}-\frac{1}{100}\right)\)
\(S=\frac{1}{3}\times\left(\frac{1}{1}-\frac{1}{100}\right)\)
\(S=\frac{1}{3}\times\frac{99}{100}\)
\(S=\frac{33}{100}\)
Cụ thể nhế
1. 0.1+0.2-0.3-04+0.5+0.6-0.7-0.8+0.9
2. 2/1x4 + 2/4x7 + ... + 2/94x97 + 2/97x100
1.
=0+0-0-0+0+0-0-0+0
=0
2.
\(=2\left(\dfrac{1}{1.4}+\dfrac{1}{4.7}+\dfrac{1}{7.10}+...+\dfrac{1}{94.97}+\dfrac{1}{97.100}\right)\)
\(=2.\dfrac{1}{3}.\left(\dfrac{3}{1.4}+\dfrac{3}{4.7}+\dfrac{3}{7.9}+...+\dfrac{3}{97.100}\right)\)
\(=\dfrac{2}{3}.\left(1-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+...+\dfrac{1}{97}-\dfrac{1}{100}\right)\)
\(=\dfrac{2}{3}.\left(1-\dfrac{1}{100}\right)\)
\(=\dfrac{2}{3}.\dfrac{99}{100}=\dfrac{2.99}{3.100}=\dfrac{1.33}{1.50}=\dfrac{33}{50}\)
bởi vì các số nào nhân với 0 cũng bằng 0 em ạ
hoặc dùng cách sau :
=0.(1+2-3-4+5+6-7-8+9)
=0.1
=0
Cụ thể nhế
1. 0.1+0.2-0.3-04+0.5+0.6-0.7-0.8+0.9
2. 2/1x4 + 2/4x7 + ... + 2/94x97 + 2/97x100 (Phân số)
1. 0,1 + 0,2 - 0,3 - 0,4 + 0,5 + 0,6 - 0,7 - 0,8 + 0,9
= ( 0,1 + 0,9 ) + ( 0,2 - 0,8 ) - ( 0,3 + 0,7 ) - ( 0,4 - 0,8 ) + 0,5
= 1 + ( - 0,6 ) - 1 - ( 0,2 ) + 0,5
= 1 + ( - 0,6 ) - 1 + 0,2 + 0,5
= [ 1 + ( - 0,6 ) ] - ( 1 - 0,2 - 0,5 )
= 0,4 - 0,3 = 0,1
tnhs giá trị của B khi B =1x 4+4x7+7x10+10x13+......+91x94+94x97
2/1x4+2/4x7+...+2/97x100
`2/[1xx4]+2/[4xx7]+...+2/[97xx100]`
`=2/3xx(3/[1xx4]+3/[4xx7]+...+3/[97xx100])`
`=2/3xx(1-1/4+1/4-1/7+...+1/97-1/100)`
`=2/3xx(1-1/100)=2/3xx99/100=33/50`
\(\dfrac{2}{1.4}+\dfrac{2}{4.7}+...+\dfrac{2}{97.100}\)
\(=\dfrac{2}{3}.\left(\dfrac{3}{1.4}+\dfrac{3}{4.7}+...+\dfrac{3}{97.100}\right)\)
\(=\dfrac{2}{3}.\dfrac{99}{100}\)
\(=\dfrac{33}{50}\)
\(\dfrac{2}{1.4}+\dfrac{2}{4.7}+...+\dfrac{2}{97.100}\)
\(=\dfrac{2}{3}.\left(\dfrac{3}{1.4}+\dfrac{3}{4.7}+...+\dfrac{3}{97.100}\right)\)
\(=\dfrac{2}{3}.\left(\dfrac{1}{1}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+...+\dfrac{1}{97}-\dfrac{1}{100}\right)\)
\(=\dfrac{2}{3}.\left(\dfrac{1}{1}-\dfrac{1}{100}\right)\)
\(=\dfrac{2}{3}.\left(\dfrac{100-1}{100}\right)\)
\(=\dfrac{2}{3}.\dfrac{99}{100}\)
\(=\dfrac{1}{1}.\dfrac{33}{50}\)
\(=\dfrac{33}{50}\)
2/1x4+2/4x7+....+2/97x100
1x4+2x5+3x6+4x7+....+98x101
4A=1.2.3.4+2.3.4(5-1)+3.4.5(6-2)+.....+98.99.100(101-97)
4A=1.2.3.4+2.3.4.5-1.2.3.4+3.4.5.6-2.3.4.5+....+98.99.100.101-97.98.99.100
4A=98.99.100.101
A=(98.99.100.101):4=24497550